I am upgrading my AMD athlon 2 x4 635 and Asus motherboard to Intel i5 3570k and GIGABYTE z77 motherboard. What I'm worried about is if windows 7 OEM that I installed to my hdd (while using my old parts) will work with the upgrades? Or will I need to reinstall or contact some part of Microsoft etc...?
AS far as I know you can reuse the OEM Version of windows BUT you will have to do a complete clean install with a Windows 7 OEM install disk. That means wiping your hard drive and performing the install as if you had built a brand new system (which you have). Backup any data that you will need before wiping the drive.
Note that OEM versions are limited to a maximum number activations so don't do this any more often than absolutely necessary.
At some point your copy of windows will give a msg saying your hardware config has changed and your product key is no longer any good but if you call the 1 800 # and be able to provide the original product key they'll hook you up with a new key, yay.